1. Field of the Invention
The present invention generally relates to a photographic camera system and, more particularly, to the camera system comprising a camera body and an interchangeable lens assembly employed in the form of a varifocal lens assembly. The varifocal lens assembly is known of a type comprising a first lens group adapted to be driven during a zooming and a second lens group adapted to be driven during a focusing and of a nature wherein the position of focus once adjusted tend to deviate with the zooming, i.e., a change in focal length.
2. Description of the Prior Art
A varifocal lens assembly has been well known in the art as one of the interchangeable photographic lens assemblies available for, for example, a single-lens reflex camera. Given the same zooming ratio, the varifocal lens assembly has some advantages as compared with the ordinary zooming lens assembly in that it can be assembled compact in size, lightweight and small in closest photo-taking distance. On the other hand, the varifocal lens assembly is known having a disadvantage in that the position of focus once adjusted tends to deviate with the zooming, that is, a change in focal length and, therefore, the zooming requires a re-adjustment in position of focus.
In view of the foregoing, attempts have been made to provide the varifocal lens assembly with means for alternately performing a predetermined amount of drive of the zooming lens group during the zooming and a compensation for a deviation in position of focus resulting from the zooming. According to the prior art such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 63-289516, published in 1988, arrangement is made that the zooming lens group can be immediately brought to a halt, when the zooming operation is interrupted, so that the focusing lens group can be driven subsequently to a correct in-focus position.
In the camera system including the varifocal lens assembly, where for the purpose of automatically compensating for the deviation in position of focus resulting from the zooming a means is provided for detecting the focal length attained subsequent to the predetermined amount of movement of the zooming lens group and for calculating the amount of defocus relative to the focal length then detected so that the compensation can be made based on the calculated amount of defocus, it has been found that the focusing tends to be delayed rather than the drive of the zooming lens group if the drive of the zooming lens groups is continued even where the deviation in focus is compensated for and that, therefore, during the drive of the zooming lens group, the defocus condition tends to continue (i.e., a follow-up compensation continues). Considering that it is not possible to predicate when the zooming operation performed at a will of a photographer is interrupted, the drive of the zooming lens group and the focusing operation are not necessarily associated with each other. Because of this, when the drive of the zooming lens group is interrupted immediately after the zooming operation has been suspended, the focusing operation will not associate with the halt of the zooming, resulting in a condition in which the position of focus is deviated.
When it comes to a photographic camera having a capability of utilizing one of a plurality of interchangeable lens assemblies, the interchangeable lens assemblies do not always include an usual lens assembly of fixed focal length and a usual zooming lens assembly of an ordinary type wherein the position of focus once adjusted will not deviate even though the zooming is performed, but include a varifocal lens assembly of the type hereinabove discussed. Some photographer may use the ordinary lens assembly on a certain occasion and the varifocal lens assembly on a different occasion, and therefore, the camera body equipped with functions necessitated only for use with the varifocal lens assembly is not practical. In other words, where the amount of defocus resulting from the zooming performed in the varifocal lens assembly is determined in reference to a ROM (read-only memory) table, the camera body will require an increased storage capacity. On the other hand, where the defocus amount is to be determined according to an arithmetic operation, an arithmetic process performed in the camera body in association with any one of AF (auto-focusing) and AE (automatic exposure) functions will be sacrificed, i.e., delayed.
In the case of the AF (auto-focusing) camera system designed to drive the focusing lens group to a correct infocus position through the detection of a focusing condition of the focusing lens group, arrangement has been made to effect a so-called low contrast scanning, i.e., an operation in which, in the event that the focus detection is impossible, the focusing lens group is driven in search for a position at which the focus detection is possible, such as disclosed in the Japanese Laid-open Patent Publication No. 59-182411.
When it comes to the camera system having the varifocal lens assembly mounted thereon, the zooming operation may often be carried out during the low contrast scanning. In such case, if the low contrast scanning is performed simultaneously with the drive of the zooming lens group in dependence on the zooming operation, not only can no proper compensation for a deviation in focus resulting from the zooming be accomplished, but also the scanning of the in-focus position as a result of the low contrast scanning cannot be performed properly. Accordingly, it is not proper to perform both of those operations simultaneously and, therefore, priority should be given to any one of them. While the drive of the zooming lens group is based on a manual operation performed by the photographer at his or her will, the low contrast scanning operation is not based on the manual operation, but is performed automatically by the camera system. Accordingly, if priority is given to the latter, no photographer's intent will be taken into in the control performed by the camera system.
Conversely, it may often occur that, in the camera system having the varifocal lens assembly mounted thereon, the focus detection becomes impossible during the drive of the zooming lens group. In such case, if the low contrast scanning is performed in a manner similar to that performed in the conventional camera system, a change in position of focus resulting from the zooming and a change in position of focus resulting from the low contrast scanning will occur simultaneously, rendering it to be difficult to effectively control the focusing lens group.